Flame-retardant non-woven fibrous webs

ABSTRACT

A nonwoven fibrous web and a method of making thereof. The nonwoven fibrous web includes greater than 0% but no greater than 30 wt % of a plurality of melt-blown fibers comprised of a crystalline (co)polymer; and at least 70 wt % of a plurality of randomly-oriented staple fibers, the plurality of randomly-oriented staple fibers including: at least 60 wt % of oxidized polyacrylonitrile fibers; and from 0 to 40 wt % of reinforcing fibers having an outer surface comprised of a (co)polymer with a melting temperature of from 100° C. to 350° C.; wherein the plurality of melt-blown fibers and the plurality of randomly-oriented staple fibers are bonded together to form a cohesive non-woven fibrous web.

FIELD OF THE INVENTION

Provided are non-woven fibrous webs and related articles including melt-blown fibers, and methods for preparing and using such fibrous webs, articles, and assemblies. The non-woven fibrous webs can be used in thermal and acoustic applications.

BACKGROUND

Non-woven articles capable of resisting high temperatures are of great interest in the aerospace, automotive, construction, transportation and electronics industries. High temperature resistant materials can be made from, for example, glass, basalt, and/or polyimide fibers. Such fibers can be formed into a non-woven web structure with a binder to provide structural integrity. Melamine foam, polyimide foam, and aramid felt materials are also known flame-retardant insulation materials.

While these materials are flame-retardant to varying degrees, these materials are generally unable to provide a combination of high surface area and high porosity achieved by webs based on melt-blown fibers. Melt blowing is a manufacturing technology capable of making fibers with diameters of less than 10 micrometers, many times finer than a human hair. Fine fibers can help achieve high performance properties in many thermal, acoustic, absorbent, and filtration applications.

The flammability of polymers that can be used in a melt blowing process also presents a significant technical challenge. Many polymeric materials are inherently flammable, and fine fibers of even less flammable polymers can be prone to burning. High temperature polymers such as polyethylene terephthalate, known for having good thermal stability, chemical resistance, and excellent mechanical properties, can be precluded from use in many applications subjected to regulated flammability standards.

SUMMARY

Improvement of flame retardancy in polymeric fibers through the use of flame-retardant additives has been reported in the literature. The halogen-containing flame-retardants are well known and play an important role due to their high efficiency while added into polymeric materials. The toxicity and corrosiveness of these flame-retardants, however, have raised environmental concerns and use of these materials have been restricted in many countries. However, it remains a challenge to make flame-retardant melt-blown non-woven articles because of poor compatibility between the melt blown polymer and flame-retardant additive, which tends to significantly increase the median fiber diameter of melt-blown materials.

Disclosed herein are non-woven fibrous combo webs made from a melt blowing microfiber fabrication process including a crystalline (co)polymer and a plurality of randomly-oriented staple fibers. The provided non-woven fibrous webs can be dimensionally stable at elevated temperatures, have very fine fibers for superior acoustic and thermal insulation properties, and display flame-retardant properties.

The product prepared by the method can have a high flame-retardant performance and can make flame-retardant ultra-fine non-woven fibrous webs with the flame-retardant property of 94ULV-0 level, VTM-0 level and FAR25-856(a) level. Some non-woven materials demonstrated superior acoustic insulation barrier properties.

In a first aspect, a nonwoven fibrous web is provided. The nonwoven fibrous web includes greater than 0% but no greater than 30 wt % of a plurality of melt-blown fibers comprised of a crystalline (co)polymer; and at least 70 wt % of a plurality of randomly-oriented staple fibers, the plurality of randomly-oriented staple fibers comprising: at least 60 wt % of oxidized polyacrylonitrile fibers; and from 0 to 40 wt % of reinforcing fibers having an outer surface comprised of a (co) polymer with a melting temperature of from 100° C. to 350° C.; wherein the plurality of melt-blown fibers and the plurality of randomly-oriented staple fibers are bonded together to form a cohesive non-woven fibrous web, optionally wherein the crystalline (co)polymer exhibits a melting temperature from 100° C. to 250° C.

In a second aspect, a nonwoven fibrous web is provided. The nonwoven fibrous web includes a cohesive nonwoven fibrous matrix comprising of a plurality of randomly-oriented staple fibers, the plurality of randomly-oriented staple fibers comprising: at least 60 wt % of oxidized polyacrylonitrile fibers; and from 0 to 40 wt % of reinforcing fibers having an outer surface comprised of a (co) polymer with a melting temperature of from 100° C. to 350° C.; wherein the plurality of randomly-oriented staple fibers are bonded together to form the cohesive non-woven fibrous matrix; and a plurality of discrete domains of at least partially melted melt-blown fibers distributed within the cohesive nonwoven fibrous matrix, wherein the at least partially melted melt-blown fibers comprise a crystalline (co)polymer, optionally wherein the crystalline (copolymer exhibits a melting temperature from 100° C. to 250° C.

In a third aspect, a method of making a cohesive nonwoven fibrous web is provided. The method includes mixing a plurality of oxidized polyacrylonitrile fibers with a plurality of reinforcing fibers to form a mixture of randomly-oriented fibers, wherein the plurality of reinforcing fibers have outer fiber surfaces comprised of a (co)polymer with a melting temperature between 100° C. and 350° C.; combining the mixture of randomly-oriented fibers with a plurality of melt-blown fibers comprised of a crystalline (co)polymer to form nonwoven fibrous web; and heating the mixture of randomly-oriented fibers combined with the plurality of melt-blown fibers to a temperature sufficient to melt the outer surfaces of the plurality of reinforcing fibers; optionally wherein the crystalline (co)polymer exhibits a melting temperature from 100° C. to 250° C.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-2 are a side, cross-sectional views of a non-woven fibrous web assemblies according to respective exemplary embodiments.

FIG. 3 is a microscopic image of a non-woven fibrous web assemblies according to one exemplary embodiment.

Repeated use of reference characters in the specification and drawings is intended to represent the same or analogous features or elements of the disclosure. It is to be understood that numerous other modifications and embodiments can be devised by those skilled in the art, which fall within the scope and spirit of the principles of the disclosure. The figures may not be drawn to scale.

DEFINITIONS

“Ambient conditions” means at 25° C. and 101.3 kPa (1 atm) pressure.

“Ambient temperature” means at 25° C.

“Basis weight” is calculated as the weight of a 10 cm×10 cm web sample multiplied by 100, and is expressed in grams per square meter (gsm).

“Bulk density” is the mass per unit volume of a non-woven fibrous web.

“Copolymer” refers to polymers made from repeat units of two or more different polymers and includes random, block and star (e.g. dendritic) copolymers.

“Dimensionally stable” refers to a structure that resists shrinkage when subjected to elevated temperatures for a given period of time, where elevated temperatures can be temperatures exceeding 80° C., 85, 90, 95, 100, 105, 110, 115, 120, 125, 130, 135, 140, 145, or 150° C.

“Die” means a processing assembly including at least one orifice for use in polymer melt processing and fiber extrusion processes, including but not limited to melt-blowing.

“Discontinuous” when used with respect to a fiber or plurality of fibers means fibers having a limited aspect ratio (e.g., a ratio of length to diameter of e.g., less than 10,000).

“Median fiber diameter” of fibers in a non-woven fibrous web is determined by producing one or more images of the fiber structure, such as by using a scanning electron microscope; measuring the transverse dimension of clearly visible fibers in the one or more images resulting in a total number of fiber diameters; and calculating the median fiber diameter based on that total number of fiber diameters.

“Melting temperature” for a polymer represents the temperature at which a polymer changes state from a solid to a liquid, and can be determined as the peak maximum of a first-heat total-heat flow plot obtained using modulated differential scanning calorimetry, occurring in the melting region of the polymer or fiber if there is only one maximum in the melting region; and, if there is more than one maximum indicating more than one melting point (e.g., because of the presence of two distinct crystalline phases), as the temperature corresponding to the highest-amplitude melting peak.

“Non-woven fibrous web” means a plurality of fibers characterized by entanglement or inter-fiber bonding of the fibers to form a sheet or mat exhibiting a structure of individual fibers or filaments which are interlaid, but not in an identifiable manner as in a knitted fabric.

“Oriented” when used with respect to a fiber means that at least portions of the polymer molecules within the fibers are aligned with the longitudinal axis of the fibers, for example, by use of a drawing process or attenuator upon a stream of fibers exiting from a die.

“Substantially” means a majority of, or mostly, as in an amount of at least 50%, 60, 70, 80, 90, 95, 96, 97, 98, 99, 99.5, 99.9, 99.99, or 99.999%, or 100%.

DETAILED DESCRIPTION

Objects and advantages of this disclosure are further illustrated by the following non-limiting examples, but materials and amounts thereof recited in these examples, as well as other conditions and details, should not be construed to unduly limit this disclosure. Herein, the terms “a”, “an”, and “the” include plural referents unless the content clearly indicates otherwise. The conjunction “or” is generally employed in its sense including “and/or” unless it is clearly indicated otherwise.

Described herein are non-woven fibrous webs, articles and assemblies thereof, and methods thereof that may be suitable for thermal and acoustic insulation. Further applications for these materials include filtration media, surgical drapes, and wipes, liquid and gas filters, garments, blankets, furniture, transportation (e.g., for aircraft, rotorcraft, trains, and automotive vehicles), upholstery, and personal protection equipment.

As illustrated in FIG. 1 , the non-woven fibrous web 100 of the present disclosure contain greater than 0% but no greater than 30 wt % of a plurality of melt-blown fibers 110 formed from a crystalline (co)polymer and at least 70 wt % of a plurality of randomly-oriented staple fibers 120. The plurality of melt-blown fibers and the plurality of randomly-oriented staple fibers are bonded together to form a cohesive non-woven fibrous web. Melt-blown fibers can increase the surface area so that to increase the acoustic absorption properties of the non-woven fibrous webs.

In some other embodiments illustrated in FIG. 2 , the non-woven fibrous web 200 of the present disclosure can contain a cohesive nonwoven fibrous matrix comprising of a plurality of randomly-oriented staple fibers 220 and a plurality of discrete domains 230 of at least partially melted melt-blown fibers distributed within the cohesive nonwoven fibrous matrix. In these embodiments, the plurality of randomly-oriented staple fibers are bonded together to form the cohesive non-woven fibrous matrix. The at least partially melted melt-blown fibers can be formed from a crystalline (co)polymer. In some embodiments, plurality of discrete domains 230 of at least partially melted melt-blown fibers can bind to the cohesive nonwoven fibrous matrix so that the discrete domains 230 of at least partially melted melt-blown fibers can be remain in the non-woven fibrous web when the non-woven fibrous web is moved, transported, or shaked.

Exemplary crystalline (co)polymer can include polyolefins such as polypropylene and polyethylene, polybutylene, polyisobutylene, poly(4-methyl-1-pentene), polyurethane, polybutene, polylactic acid, polyphenylene sulfide, polysulfone, liquid crystalline polymer, polyethylene-co-vinylacetate, polyacrylonitrile, cyclic polyolefins, along with copolymers and blends thereof.

The crystalline (co)polymer preferably has an intrinsic viscosity (IV) suitable for preparation of fine fibers in a melt blowing process. The intrinsic viscosity of a given polymer is defined as the limiting value of the reduced viscosity, or inherent viscosity, at infinite dilution of the polymer. This parameter can be correlated with the melting point, crystallinity and tensile strength of the polymer. Various methods can be used to determine intrinsic viscosity. For example, intrinsic viscosity can be measured using a Ubbelohde viscometer or obtained by measuring melt flow index of the polymer using an extrusion plastometer and correlating the melt flow index to intrinsic viscosity based on internal calibration curves of the equipment. Intrinsic viscosity can be in the range of from 0.4 to 0.7, from 0.4 to 0.6, from 0.4 to 0.5, or in some embodiments, less than, equal to, or greater than 0.4, 0.42, 0.45, 0.47, 0.5, 0.52, 0.55, 0.57, 0.6, 0.62, 0.65, 0.67, or 0.7.

The crystalline (co)polymer can exhibit a melting temperature from 100° C. to 250° C., from 100° C. to 200° C., or in some embodiments, less than, equal to, or greater than 100° C., 120° C., 140° C., 150° C., 160° C., 170° C., 180° C., 190° C., 200° C., 210° C., 220° C., 230° C., 240° C., or 250° C.

Additional details on crystalline (co)polymer useful for making non-woven fibrous webs can be found in, for example, U.S. Pat. Nos. 7,757,811 (Fox et al.) and 9,194,065 (Moore et al.).

The median fiber diameter of the plurality of melt-blown fibers in the non-woven fibrous web can be engineered to provide properties desired in the end application. For acoustic absorbers, for example, it can be desirable for the median fiber diameter to be as small as possible to obtain the greatest surface area per unit volume. The minimum fiber diameters achievable are at least partially dependent on the melt viscosities of the polymers used to form the fibers.

In the provided webs, these fibers can have a median diameter of from 0.2 micrometers to 20 micrometers, from 0.5 micrometers to 15 micrometers, from 1 micrometers to 20 micrometers, from 1 micrometers to 10 micrometers, or in some embodiments, less than, equal to, or greater than 0.2 micrometers, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, 8.5, 9, 9.5, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20 micrometers.

Fine melt-blown fibers are of great technical benefit in thermal and acoustic insulation applications. In acoustics, the high surface area per unit volume results in enhanced viscous dissipation of sound energy within the non-woven fibrous web. In thermal applications, the fine fibers trap air and block radiant heat loss, making the non-woven fibrous web an effective insulator.

Based on the nature of the crystalline (co)polymer used, manufacturing process, and presence of randomly-oriented staple fibers, the provided non-woven fibrous webs can have a wide range of bulk densities. The provided webs can display a bulk density of from 1 kg/m³ to 1000 kg/m³, 1 kg/m³ to 100 kg/m³, 1 kg/m³ to 50 kg/m³, or in some embodiments, less than, equal to, or greater than 1 kg/m³, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 15, 17, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 120, 150, 170, 200, 250, 300, 350, 400, 450, 500, 600, 700, 800, 900, or 1000 kg/m³.

Average bulk density has significant bearing on the insulation performance of the non-woven fibrous web. When the average bulk density of the non-woven fibrous web is significantly higher than 50 kg/m³, a significant amount of heat can be transmitted through the insulator by thermal conduction through the fibers themselves. When the average bulk density is significantly below 15 kg/m³, heat conduction through the fibers is small but convective heat transfer can become significant. Further reduction of average bulk density can also degrade strength of the non-woven fibrous web, which is not desirable.

In exemplary embodiments, the non-woven fibrous web has a basis weight of from 100 gsm to 500 gsm, 150 gsm to 450 gsm, 200 gsm to 400 gsm, or in some embodiments less than, equal to, or greater than 100, 150, 200, 250, 300, 350, 400, or 450 gsm.

In exemplary embodiments, the non-woven fibrous web has a thermal conductivity coefficient of less than 0.04, 0.03, 0.02, 0.01 W/K-m at 25° C. in its relaxed configuration.

In exemplary embodiments, the non-woven fibrous web has a sound absorption coefficient of 0.2 to 0.99, or in some embodiments less than, equal to, or greater than 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 or 0.99 at 1000 Hz at normal sound incidance.

The plurality of randomly-oriented staple fibers can be present in any amount suitable to obtain the degree of flame retardancy desired and obtain acceptable fibers diameter in the melt blowing manufacturing process for a given raw materials cost. While not particularly restricted, the amount of randomly-oriented staple fibers present can be in an amount ranging greater than 70 wt %, 75, 80, 90 or 95 wt %, based on the overall weight of the nonwoven fibrous web. Often, randomly-oriented staple fibers are significantly thicker than the melt-blown fibers to provide mechanical reinforcement. Incorporation of randomly-oriented staple fibers can provide many potential benefits to the web, including increased loft (or lower density), resilience, and/or strength. These fibers can also improve the thermal or acoustic insulation properties of the web.

In some cases, randomly-oriented staple fibers can be made from non-meltable materials. Non-meltable materials, do not become a liquid at any temperature and may be polymeric or non-polymeric. Many of these materials do not melt because they oxidize or otherwise degrade first when heated in the presence of air. Non-meltable polymeric fibers can include carbon fibers, carbon fiber precursors, or a combination thereof. If incorporated in sufficient amounts, these randomly-oriented staple fibers can significantly enhance the flame retardancy of the overall web.

Carbon fiber precursors can include oxidized acrylic precursors, such as oxidized polyacrylonitrile. Polyacrylonitrile is a useful acrylic precursor that can be used widely to produce the carbon fibers. In some embodiments, the polyacrylonitrile contains more than 60 wt %, 70 wt %, more than 75 wt %, more than 80 wt %, or more than 85 wt % acrylonitrile repeat units.

In a preferred embodiment, the non-meltable fibers are comprised of oxidized polyacrylonitrile fibers. The oxidized polyacrylonitrile fibers can include, for example, those available under the trade designations PYRON (Zoltek Corporation, Bridgeton, Mo.) and PANOX (SGL Group, Meitingen, Germany).

Oxidized polyacrylonitrile fibers can be made from precursor fibers containing a copolymer of acrylonitrile and one or more co-monomers. Useful co-monomers include, for example, methyl methacrylate, methyl acrylate, vinyl acetate, and vinyl chloride. The co-monomer(s) may be present in an amount of up to 15 wt %, 14 wt %, 13 wt %, 12 wt %, 11 wt %, 10 wt %, 9 wt %, or 8 wt %, relative to the overall weight of the monomer mixture prior to copolymerization.

The precursor fibers can be oxidized in a multi-step process. The fibers are initially stabilized at high temperatures to prevent melting or fusion of the fibers, then carbonized to eliminate the non-carbon elements, and finally graphitized at even higher temperatures to enhance the mechanical properties of the fibers. Oxidized polyacrylonitrile fibers include polyacrylonitrile fibers that are either partially or fully oxidized, and may or may not be graphitized.

The randomly-oriented staple fibers can have a fiber diameter and length that enables the fibers to become entangled within the non-woven fibrous web. The fibers, however, are preferably not so thin that web strength is unduly compromised. For most applications, the randomly-oriented staple fibers can have a median fiber diameter in the range from 5 micrometers to 1000 micrometers, from 5 micrometers to 300 micrometers, from 5 micrometers to 100 micrometers, or in some embodiments, less than, equal to, or greater than 5 micrometer, 10, 11, 12, 13, 15, 17, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 150, 200, 250, 300, 350, 400, 450, 500, 600, 700, 800, 900, or 1000 micrometers.

Use of relatively long fibers can reduce fiber shedding and further enhance strength of the non-woven fibrous web along transverse directions. The randomly-oriented staple fibers can have an average fiber length in the range from 3 millimeters to 100 millimeters, from 15 millimeters to 100 millimeters, from 25 millimeters to 75 millimeters, or in some embodiments, less than, equal to, or greater than 3, 5, 10 millimeters, 12, 15, 17, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, or 75 millimeters.

In some embodiments, the plurality of randomly-oriented staple fibers can include from 0 to 40 wt % of reinforcing fibers. The reinforcing fibers may include binder fibers, which have a sufficiently low melting temperature. Binder fibers are generally polymeric, and may have uniform composition or contain two or more components. In some embodiments, the binder fibers are bi-component fibers comprised of a core polymer that extends along the axis of the fibers and is surrounded by a cylindrical shell polymer. The shell polymer can have a melting temperature less than that of the core polymer. The reinforcing fibers can include at least one of monocomponent or multi-component fibers. In some embodiments, the reinforcing fiber can include polyethylene terephthalate, polyphenylene sulfide, poly-aramide, polylactic acid. In some embodiments, the reinforcing fibers can be multicomponent fibers having an outer shealth comprising polyolefin. In some embodiments, the polyolefin can be selected from the group consisting of polyethylene fibers, polypropylene fibers, polybutylene fibers, polyisobutylene fibers, poly(4-methyl-1-pentene), and combinations thereof.

As used herein, however, “melting” refers to a gradual transformation of the fibers or, in the case of a bi-component shell/core fiber, an outer surface of the fiber, at elevated temperatures at which the polyester becomes sufficiently soft and tacky to bond to other fibers with which it comes into contact, including oxidized polyacrylonitrile fibers and any other binder fibers having its same characteristics and, as described above, which may have a higher or lower melting temperature.

Useful binder fibers have an outer surface comprised of a (co)polymer having a melting temperature of from 100° C. to 450° C., 100° C. to 350° C. or in some embodiments, less than, equal to, or greater than, 100° C., 105, 110, 115, 120, 125, 130, 135, 140, 145, 150, 160, 170, 180, 190, 200, 210, 220, 230, 240, 250, 260, 270, 280, 290, 300, 325, 350, 375, 400, 425° C.

To maximize flame retardancy in the finished non-woven fibrous web, it can be advantageous for any meltable fibers present to be blended with flame-retardant randomly-oriented staple fibers.

The non-woven fibrous webs of the present disclosure can be found in articles and assemblies deployed in any of a number of thermal and acoustic applications. Exemplary thermal and acoustic applications include, for example, battery compartments for electric vehicles, engine compartments, automotive vehicle doors and ceilings, railway car insulation applications such as under window and floor treatments in trains, automotive trunks, automotive under hood applications, building and utility wraps, furniture upholstery, exit walkways on aircraft or in buildings, heating ventilation and air conditioning (HVAC) systems, rotorcraft cabins, and aerospace fuselages.

The provided non-woven fibrous webs and assemblies display numerous advantages, at least some of which are unexpected. These materials can be used in thermal and acoustic insulation applications at high temperatures where conventional insulation materials would thermally degrade or fail. Particularly demanding are automotive and aerospace vehicle applications, where insulation materials operate in environments that are not only noisy but can reach extreme temperatures.

The provided webs are capable of passing standardized flammability and flame propagation tests used in regulated industries such as automotive and aerospace vehicles. In some embodiments, a 20-millimeter thick sample of the non-woven fibrous web is capable of passing one or more of the flammability tests UL 94 V0, FAR 25.853(a), and FAR 25.856(a).

The provided non-woven fibrous webs can provide acoustic absorption over a wide range of frequencies. The ratio of the absorbed sound energy to the incident energy represents a sound absorption coefficient. A 400 gsm sample of the provided web can, in various embodiments, display a sound absorption coefficient that is greater than 0.2, greater than 0.3, greater than 0.4, or in some embodiments, less than, equal to, or greater than 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 0.55, 0.6, 0.65, 0.7 or 0.8, 0.9 at normal sound incidence, for example, an acoustic frequency of 1000 Hz when tested under ambient conditions.

Methods of Manufacture

Melt-blown fibers are formed from a crystalline (co)polymer using a melt blowing process. In a melt-blowing process, one or more a crystalline (co)polymer streams are extruded through a die containing closely arranged orifices and attenuated by convergent streams of hot air at high velocities to form fine fibers. These fine fibers can be collected on a surface to provide a melt-blown non-woven fibrous web.

Depending on the degree of solidification from the molten state, the collected fibers can be semi-continuous or essentially discontinuous. In certain exemplary embodiments, the melt-blown fibers of the present disclosure may be oriented on a molecular level. As an option, at least some of the plurality of fibers in the non-woven fibrous web are physically bonded to each other through heat and pressure applied or by calendering.

Other techniques for bonding the fibers is taught in, for example, U.S. Patent Publication No. 2008/0038976 (Berrigan et al.) and U.S. Pat. No. 7,279,440 (Berrigan et al.). One technique involves subjecting the collected web of fibers and fibers to a controlled heating and quenching operation that includes forcefully passing through the web a gaseous stream heated to a temperature sufficient to soften the fibers sufficiently to cause the fibers to bond together at points of fiber intersection, where the heated stream is applied for a time period too short to wholly melt the fibers, and then immediately forcefully passing through the web a gaseous stream at a temperature at least 50° C. less than the heated stream to quench the fibers.

In some embodiments, the process includes providing to a melt-blowing die a molten stream of a thermoplastic material including at least one thermoplastic semicrystalline polymer and subjecting the at least one fiber immediately upon exiting the melt-blowing die and prior to collection as a non-woven fibrous web on a collector, to a controlled in-flight heat treatment operation at a temperature below a melting temperature of the at least one thermoplastic semi-crystalline polymer for a time sufficient for the non-woven fibrous web to exhibit a Shrinkage less than a Shrinkage measured on an identically-prepared structure that is not subjected to the controlled in-flight heat treatment operation.

Further options and advantages associated with the in-flight heat treatment operation and of non-woven fibrous webs made therefrom, are described in detail in U.S. Patent Publication Nos. 2016/0298266 (Zillig et al.) and International Patent Publication No. WO 2018/0126085 (Ren et al.).

Randomly-oriented staple fibers, if present, can be generally obtained from bulk fibers. One technical challenge with incorporating randomly-oriented staple fibers into a non-woven fibrous web arises out of difficulties in handling and feeding these fibers into a large-scale manufacturing process. This is especially problematic when dealing with non-meltable fibers such as oxidized polyacrylonitrile fibers which tend to be weakly entangled and can unravel easily in bulk form.

This difficulty can be overcome by using a blend of randomly-oriented staple fibers that includes both non-meltable fibers and thermoplastic reinforcement fibers having a significantly larger diameter. In an exemplary embodiment, a pre-formed feed web could be obtained by blending oxidized polyacrylonitrile fibers having a median fiber diameter in the range of 5 micrometers to 15 micrometers and polyethylene terephthalate staple fibers having a median fiber diameter in the range of 30 micrometers to 60 micrometers. Here, the inclusion of polyethylene terephthalate staple fibers provides significant strength to the feed web.

To provide further strength to the feed web, the blended fibers can be substantially entangled with each other using a process such as needle tacking or hydroentangling. Optionally, these fibers are crimped to provide greater web thickness and reduce bulk density. Details in the process of making these webs are described in co-pending International Patent Application No. PCT/CN2017/110372 (Cai, et al.).

Once a feed web with suitable strength has been made, it can be transferred to a separate melt blowing process, where into a multicomponent melt-blown microfiber web can be made according to any of the methods described in U.S. Pat. Nos. 4,118,531 (Hauser); 5,298,694 (Thompson et al.); 5,773,375 (Swan, et al.); 5,961,905 (Swan, et al.); and 7,476,632 (Olson, et al.).

In an exemplary process, flow stream of a crystalline (co)polymer is fed into a manifold. The flow stream is then fed into the die and through a series of die orifices. Air slots are disposed on either side of the die orifices and direct uniform heated air at high velocities at the extruded melt stream. The hot high velocity air draws and attenuates the extruded polymeric material which solidifies after traveling a relatively short distance from the die. The high velocity air becomes turbulent between the die and the collector surface causing the melt-blown fibers entrained in the airstream to become intimately mixed.

In mid-flight between the melt blowing die and the collector, this mixed stream of melt-blown fibers is blended with randomly-oriented staple fibers from the feed web, which can be continuously plucked from a leading edge of the feed web by a rotating lickerin roll. The plucked randomly-oriented staple fibers are directed into the turbulent airstream, where it is uniformly dispersed and distributed along with the melt-blown fibers and eventually gathered on a perforated collector drum or mesh belt to provide a coherent non-woven web.

As a further option, a subsequent activation process can be used to bind meltable fibers to each other at points of inter-fiber contact. This can be achieved by passing the web through an oven heated to a temperature at or above the softening point of the meltable fibers or the meltable sheath of the shear/core plurality of reinforcing fibers, if used. Such inter-fiber bonded webs can have increased physical integrity and tensile strength as a result of the additional bonds formed between fibers.

While not intended to be exhaustive, further exemplary embodiments are provided below:

1. A nonwoven fibrous web comprising greater than 0% but no greater than 30 wt % of a plurality of melt-blown fibers comprised of a crystalline (co)polymer; and at least 70 wt % of a plurality of randomly-oriented staple fibers, the plurality of randomly-oriented staple fibers comprising: at least 60 wt % of oxidized polyacrylonitrile fibers; and from 0 to 40 wt % of reinforcing fibers having an outer surface comprised of a (co)polymer with a melting temperature of from 100° C. to 350° C.; wherein the plurality of melt-blown fibers and the plurality of randomly-oriented staple fibers are bonded together to form a cohesive non-woven fibrous web, optionally wherein the crystalline (co)polymer exhibits a melting temperature from 100° C. to 250° C. 2. The nonwoven fibrous web of embodiment 1, wherein at least one of the plurality of reinforcing comprise polyolefin fibers. 3. The nonwoven fibrous web of embodiment 2, wherein the polyolefin fibers are selected from the group consisting of polyethylene fibers, polypropylene fibers, polybutylene fibers, polyisobutylene fibers, poly(4-methyl-1-pentene), and combinations thereof. 4. The nonwoven fibrous web of any one of embodiments 1-3, wherein the crystalline (co)polymer is selected from the group consisting of polyethylene, polypropylene, polybutylene, polyisobutylene, poly(4-methyl-1-pentene), and combinations thereof. 5. The nonwoven fibrous web of any one of embodiments 1-4, wherein the non-woven fibrous web passes the UL-94V0 flame test. 6. The nonwoven fibrous web of any one of embodiments 1-5, wherein the oxidized polyacrylonitrile fibers have a median Effective Fiber Diameter of from 5 micrometers to 100 micrometers. 7. The nonwoven fibrous web of any one of embodiments 1-6, wherein the melt-blown fibers have a median Effective Fiber Diameter of from 0.1 to 20 micrometers. 8. The nonwoven fibrous web of any one of embodiments 1-7, wherein the non-woven fibrous web has a thermal conductivity coefficient of less than 0.04 W/K-m at 25° C. in its relaxed configuration. 9. The nonwoven fibrous web of any one of embodiments 1-8, wherein the non-woven fibrous web has a sound absorption coefficient of greater than 0.08 at 1000 Hz at normal sound incidence. 10. The nonwoven fibrous web of any one of embodiments 1-9, wherein the non-woven fibrous web has a base weight of from 100 gsm to 500 gsm. 11. An article comprising the nonwoven fibrous web of any one of embodiments 1-10, wherein the article is an acoustic insulation article, a thermal insulation article, or combinations thereof. 12. A non-woven fibrous web comprising a cohesive nonwoven fibrous matrix comprising of a plurality of randomly-oriented staple fibers, the plurality of randomly-oriented staple fibers comprising: at least 60 wt % of oxidized polyacrylonitrile fibers; and from 0 to 40 wt % of reinforcing fibers having an outer surface comprised of a (co)polymer with a melting temperature of from 100° C. to 350° C.; wherein the plurality of randomly-oriented staple fibers are bonded together to form the cohesive non-woven fibrous matrix; and a plurality of discrete domains of at least partially melted melt-blown fibers distributed within the cohesive nonwoven fibrous matrix, wherein the at least partially melted melt-blown fibers comprise a crystalline (co)polymer, optionally wherein the crystalline (co)polymer exhibits a melting temperature from 100° C. to 250° C. 13. A method of making a cohesive nonwoven fibrous web comprising: mixing a plurality of oxidized polyacrylonitrile fibers with a plurality of reinforcing fibers to form a mixture of randomly-oriented fibers, wherein the plurality of reinforcing fibers have outer fiber surfaces comprised of a (co)polymer with a melting temperature between 100° C. and 350° C.; combining the mixture of randomly-oriented fibers with a plurality of melt-blown fibers comprised of a crystalline (co)polymer to form nonwoven fibrous web; and heating the mixture of randomly-oriented fibers combined with the plurality of melt-blown fibers to a temperature sufficient to melt the outer surfaces of the plurality of reinforcing fibers; optionally wherein the crystalline (co)polymer exhibits a melting temperature from 100° C. to 250° C. 14. The method of embodiment 13, wherein the crystalline (co)polymer is selected from the group consisting of polyethylene, polypropylene, polybutylene, polyisobutylene, poly(4-methyl-1-pentene), and combinations thereof. 15. The method of embodiment 13 or 14, further comprising heating the nonwoven fibrous web above the melting temperature of the (co)polymer.

EXAMPLES

Objects and advantages of this disclosure are further illustrated by the following non-limiting examples, but the particular materials and amounts thereof recited in these examples, as well as other conditions and details, should not be construed to unduly limit this disclosure.

TABLE 1 Materials Designation Description Source OPAN Oxidized polyacrylonitrile staple Zoltek ™ Corporation fiber, 1.7 dTex available under the (wholly owned subsidiary trade designation “OX” of Toray Group), Bridgeton, MO. United States 3860X High purity, high flow homopolymer Total Petrochemicals, available under the trade Houston, TX. United designation “3860X” States T270 A high temperature flame retardant Trevira GmbH, polyethylene terephthalate staple Hattersheim, Germany. fiber, 6.5 dTex, available under the trade designation “TREVIRA 270” T276 A low temperature flame retardant Trevira GmbH, polyethylene terephthalate staple Hattersheim, Germany. fiber, 3.3 dTex, available under the trade designation “TREVIRA 276” LENZING Flame resistant cellulosic fiber, Ivodex Inc, Toronto, 3.3 dTex available under the Ontario, Canada designation “Lenzing FR Fiber”

Test Methods

UL94-V0 Flame Test: Reference to UL94-V0 standard with flame height 20-mm, bottom edge of the sample 10-mm into the flame and burn twice at 10 seconds each. A flame propagation height under 125-mm (5 inches) was considered a pass. Normal Incident Acoustical Absorption Test: Normal incident acoustical absorption was tested according to ASTM E1050-12, “Standard Test Method for Impedance and Absorption of Acoustical Materials Using a Tube, Two Microphones and a Digital Frequency Analysis System”. An “IMPEDANCE TUBE KIT (50 HZ-6.4 KHZ) TYPE 4206” available from Brüel & Kjær (Denmark) was used. The normal incident absorption coefficient was reported, using the abbreviation “a”. Thermal Conductivity Test: The methods of ASTM D5736 and ASTM D1518-85R03 were followed. Gap measurements were set at 6.28 mm and 6.30 mm.

Examples 1-5 (EX1-EX5) Step 1: Staple Fiber Preparation

Blends of OPAN, LENZING, T270, and T270 as represented in Table 1 were combined on a RANDO-WEBBER from Rando Machine Corporation of Macedon, N.Y. United States to produce staple fiber webs. The basis weight for each sample was 130 gsm. The webs were then folded (changing basis weight to 260 gsm) and conveyed into a Dilo Needle Loom, Model DI-Loom OD-1 6 from Eberbach, Germany having a needleboard array of 23 rows of 75 needles/row where the rows were slightly offset to randomize the pattern. The needles were Foster 20 3-22-1.5B needles. The array was roughly 17.8 cm (7 inches) deep in the machine direction and nominally 61 cm (24 inches) wide with needle spacings of roughly 7.6-mm (0.30 inch). The needleboard was operated at 91 strokes/minute to entangle and compact the web to a roughly 5.1-mm (0.20 inch) thickness. Step 2: Blend with Melt Blown Fibers

Sample webs were produced by processes and techniques described in the commonly owned PCT Patent Publication No. WO 2015/080913 (Zillig et al) with exception that the in-flight heat treatment step was not performed. Sample fiber weight percentages to produce the webs and the resulting basis weights are represented in Table 1. Air heater (operating at 110 kW obtained from Sylvania of Danvers, Mass. United States) temperatures were set at 375° C. (707° F.) and die/neck tube temperature were set at 320° C. (608° F.).

Step 3: Testing

Examples 1-5 underwent UL94-V0 Flame testing and the results are represented in Table 2. Example 4 also underwent Thermal Conductivity and Normal Incident Acoustical Absorption testing. At 11.5° C. (52.7° F.) and 20° C. (68° F.), thermal conductivity results were 0.0270 W/K·m and 0.0283 W/K·m respectfully. Normal Incident Acoustical Absorption test results are represented in Table 3.

TABLE 1 Sample Web Compositions Melt Blown Staple Basis Weight Fiber Fiber (gsm) EX1 24 wt. % 76 wt. % (80 wt. % OPAN 120 ± 10% (100 wt. % 3860X) and 20 wt. % T276) EX2 25 wt. % 75 wt. % (100 wt. % 120 ± 10% (100 wt. % 3860X) OPAN) EX3 24 wt. % 76 wt. % (80 wt. % OPAN 120 ± 10% (100 wt. % 3860X) and 20 wt. % T270) EX4 22 wt. % 78 wt. % (80 wt. % OPAN 147 ± 10% (100 wt. % 3860X) and 20 wt. % T270) EX5 22 wt. % 78 wt. % (80 wt. % 131 ± 10% (100 wt. % 3860X) LENZING and 20 wt. % T270)

TABLE 2 UL94-V0 Test Results Pass/Fail EX1 Fail EX2 Fail EX3 Fail EX4 Pass EX5 Fail

Comparative Example 1 (CE1)

3M THINSULATE AU0920 (107 gsm, 10 mm thickness) underwent Normal Incident Acoustical Absorption testing and the results are represented in Table 3.

Comparative Example 2 (CE2)

3M THINSULATE TAI 2099 (200 gsm, 8 mm thickness) obtained from 3M Company of St. Paul, Minn. United States underwent Normal Incident Acoustical Absorption testing and the results are represented in Table 3.

TABLE 3 Normal Incident Acoustical Absorption Test Results Frequency Absorption Coefficient “α” (Hz) CE1 CE2 EX4 160 2.81E−02 2.45E−02 0.00E+00 200 2.53E−02 2.78E−02 4.19E−02 250 3.58E−02 3.27E−02 5.39E−02 315 3.38E−02 3.78E−02 7.74E−02 400 4.20E−02 4.10E−02 7.57E−02 500 5.40E−02 5.04E−02 9.28E−02 630 6.16E−02 6.37E−02 1.03E−01 800 7.44E−02 8.13E−02 1.22E−01 1000 8.49E−02 1.09E−01 1.43E−01 1250 1.18E−01 1.39E−01 1.63E−01 1600 1.55E−01 1.88E−01 1.82E−01 2000 2.07E−01 2.62E−01 2.21E−01 2500 2.80E−01 3.61E−01 2.76E−01 3150 3.76E−01 4.97E−01 3.38E−01 4000 5.21E−01 6.48E−01 3.94E−01 5000 6.53E−01 7.85E−01 5.04E−01

Examples 6 (EX6)

A sample identically constructed as described in Example 4 was heated in an oven for three minutes at 232.2° C. (450° F.). A microscopic image highlighting the randomly-oriented staple fibers bonded to the melt-blown fibers to form a cohesive non-woven fibrous web was captured immediately upon removal from the oven and the image is represented in FIG. 3 .

All cited references, patents, and patent applications in the above application for letters patent are herein incorporated by reference in their entirety in a consistent manner. In the event of inconsistencies or contradictions between portions of the incorporated references and this application, the information in the preceding description shall control. The preceding description, given in order to enable one of ordinary skill in the art to practice the claimed disclosure, is not to be construed as limiting the scope of the disclosure, which is defined by the claims and all equivalents thereto. 

1. A nonwoven fibrous web comprising greater than 0% but no greater than 30 wt % of a plurality of melt-blown fibers comprised of a crystalline (co)polymer; and at least 70 wt % of a plurality of randomly-oriented staple fibers, the plurality of randomly-oriented staple fibers comprising: at least 60 wt % of oxidized polyacrylonitrile fibers; and from 0 to 40 wt % of reinforcing fibers having an outer surface comprised of a (co)polymer with a melting temperature of from 100° C. to 350° C.; wherein the plurality of melt-blown fibers and the plurality of randomly-oriented staple fibers are bonded together to form a cohesive non-woven fibrous web, optionally wherein the crystalline (co)polymer exhibits a melting temperature from 100° C. to 250° C.
 2. The nonwoven fibrous web of claim 1, wherein at least one of the plurality of reinforcing comprise polyolefin fibers.
 3. The nonwoven fibrous web of claim 2, wherein the polyolefin fibers are selected from the group consisting of polyethylene fibers, polypropylene fibers, polybutylene fibers, polyisobutylene fibers, poly(4-methyl-1-pentene), and combinations thereof.
 4. The nonwoven fibrous web of claim 1, wherein the crystalline (co)polymer is selected from the group consisting of polyethylene, polypropylene, polybutylene, polyisobutylene, poly(4-methyl-1-pentene), and combinations thereof.
 5. The nonwoven fibrous web of claim 1, wherein the non-woven fibrous web passes the UL-94V0 flame test.
 6. The nonwoven fibrous web of claim 1, wherein the oxidized polyacrylonitrile fibers have a median Effective Fiber Diameter of from 5 micrometers to 100 micrometers.
 7. The nonwoven fibrous web of claim 1, wherein the melt-blown fibers have a median Effective Fiber Diameter of from 0.1 to 20 micrometers.
 8. The nonwoven fibrous web of claim 1, wherein the non-woven fibrous web has a thermal conductivity coefficient of less than 0.04 W/K-m at 25° C. in its relaxed configuration.
 9. The nonwoven fibrous web of claim 1, wherein the non-woven fibrous web has a sound absorption coefficient of greater than 0.08 at 1000 Hz at normal sound incidence.
 10. The nonwoven fibrous web of claim 1, wherein the non-woven fibrous web has a base weight of from 100 gsm to 500 gsm.
 11. An article comprising the nonwoven fibrous web of claim 1, wherein the article is an acoustic insulation article, a thermal insulation article, or combinations thereof.
 12. A non-woven fibrous web comprising a cohesive nonwoven fibrous matrix comprising of a plurality of randomly-oriented staple fibers, the plurality of randomly-oriented staple fibers comprising: at least 60 wt % of oxidized polyacrylonitrile fibers; and from 0 to 40 wt % of reinforcing fibers having an outer surface comprised of a (co) polymer with a melting temperature of from 100° C. to 350° C.; wherein the plurality of randomly-oriented staple fibers are bonded together to form the cohesive non-woven fibrous matrix; and a plurality of discrete domains of at least partially melted melt-blown fibers distributed within the cohesive nonwoven fibrous matrix, wherein the at least partially melted melt-blown fibers comprise a crystalline (co)polymer, optionally wherein the crystalline (co)polymer exhibits a melting temperature from 100° C. to 250° C.
 13. A method of making a cohesive nonwoven fibrous web comprising: mixing a plurality of oxidized polyacrylonitrile fibers with a plurality of reinforcing fibers to form a mixture of randomly-oriented fibers, wherein the plurality of reinforcing fibers have outer fiber surfaces comprised of a (co)polymer with a melting temperature between 100° C. and 350° C.; combining the mixture of randomly-oriented fibers with a plurality of melt-blown fibers comprised of a crystalline (co)polymer to form nonwoven fibrous web; and heating the mixture of randomly-oriented fibers combined with the plurality of melt-blown fibers to a temperature sufficient to melt the outer surfaces of the plurality of reinforcing fibers; optionally wherein the crystalline (co)polymer exhibits a melting temperature from 100° C. to 250° C.
 14. The method of claim 13, wherein the crystalline (co)polymer is selected from the group consisting of polyethylene, polypropylene, polybutylene, polyisobutylene, poly(4-methyl-1-pentene), and combinations thereof.
 15. The method of claim 13, further comprising heating the nonwoven fibrous web above the melting temperature of the (co)polymer. 